THE NEUROCHEMICAL CODING AND PROJECTIONS OF CIRCULAR MUSCLE MOTOR-NEURONS IN THE HUMAN COLON

Background & Aims: Enteric neurons can be characterized by their chemi cal coding, projections, and morphology. The aim of this study was to describe the different classes of human colonic circular muscle motor neurons. Methods: Human colonic circular muscle motor neurons were ide ntified by retrograde tracing with 1,1'-didodecyl 3,3,3',3'-indocarboc yanine perchlorate (Dil) applied to the circular muscle layer. Whole-m ount preparations of the myenteric plexus were then double-labeled wit h antisera to choline acetyltransferase (ChAT) and/or nitric oxide syn thase (NOS), or NOS and vasoactive intestinal peptide (VIP), and the p osition and immunoreactivity of Dil-filled neurons were recorded. Resu lts: Fifty-two percent of all Dil-filled neurons were ChAT immunoreact ive, and 86% of these projected up to 11 mm orally, with 14% projectin g short distances anally. Forty-eight percent of the Dil-filled neuron s were NOS immunoreactive, and 77% of these projected up to 19 mm anal ly, with 23% projecting no more than 6 mm orally. A subpopulation of t hese NOS-immunoreactive motor neurons were also VIP-immunoreactive. A small population of myenteric neurons was immunoreactive for both ChAT and NOS, but none projected to the circular muscle. NOS-immunoreactiv e motor neurons projected for longer distances than those with ChAT im munoreactivity and were larger. Conclusions: There are two classes of human colonic motor neurons: one is excitatory (ChAT-immunoreactive) a nd mainly projects orally and the other is inhibitory (NOS +/- VIP imm unoreactive) and projects preferentially anally.